The aim of our study is to determine the immunogenicity of recombinant hepatitis B vaccine when administered intramuscularly to premature infants. Vaccinees will be recruited from CHOP, HUP, and Lankenau Hospital. After consent, participants will receive Recombivax HB at times 0, 1 month, and 6 months. Time 0 will occur when the child is medically stable and approaching hospital discharge. Blood will be obtained at time 0 for HBsAg, anti-HBS, and anti-HBc. Blood will be obtained for anti-HBs at times 1 and 6 months.